The present invention relates to a method for the preparation of a pitch material for spinning into carbon fibers or, more particularly to a method for the preparation of a pitch material for carbon fibers starting from coal tar or coal tar pitch as a kind of heavy bituminous material and comprising two steps composed of the first step for the pretreatment of the starting material and the second step for the heat treatment of the pretreated material at a high temperature of 430.degree. C. or higher within a relatively short time of 60 minutes or less. In particular, the pretreatment in the first step is carried out characteristically by heating the starting material at a temperature of 350.degree. to 500.degree. C. in the presence of an aromatic oil, optionally, containing catalyst for catalytic cracking.
Needless to say, carbon fibers are very promising as a future material usable in large quantities as a heat-insulating material, component of structural bodies including various sporting goods by utilizing their excellent properties such as the outstandingly high tensile strength, elastic modulus, heat resistance, resistance against chemicals and electric conductivity.
The carbon fibers constituting the major current of the products are produced from polyacrylonitrile (referred to as PAN hereinbelow) fibers or pitches. The PAN-based carbon fibers are known to have extremely high tensile strength and elastic modulus which sometimes exceed 350 kg/mm.sup.2 and 40 tons/mm.sup.2, respectively, and the highest values recently attained are about 500 kg/mm.sup.2 of the tensile strength and about 2% of elongation. The problems in the PAN-based carbon fibers is the low yield of the product from the starting material which is usually 60% or smaller and the cost of the product due to the high production costs. The pitch-based carbon fibers are inferior in the properties to the PAN-based ones and the current products are the low-strength grade ones for general purpose having a tensile strength of 100 kg/mm.sup.2 or smaller and high-strength grade carbon fibers of the so-called HP-grade are not being produced from pitches. Very recently, production of high-modulus pitch-based carbon fibers having a tensile strength of about 200 kg/mm.sup.2 has been started although the quality of the product is not quite satisfactory.
In order to produce carbon fibers of HP-grade from a pitch as the starting material, it is essential, as is known, that the pitch for spinning is a carbonaceous mesophase pitch having optical anisotropy. The reason therefor is as follows. When a pitch is heated and carbonized through the reactions of thermal decomposition and thermal polymerization, a so-called liquid-phase carbonization process proceeds in which an optically anisotropic material, i.e. mesophase, is formed in the basically isotropic pitch and the mesophase propagates over whole volume of the pitch. Such a process of liquid-phase carbonization can take place only in a considerably large volume of the pitch and the process taking place in a microregion such as a fiber is different resulting in the carbonization being concluded without the molecular movement due to the disturbed mobility of the molecules forming the pitch material. Such a process is similar to the so-called solid-carbonization process. Therefore, the degree of the molecular orientation in the pitch fibers, i.e. the fibrous pitch obtained by spinning, is the determining factor for the grade of the resultant carbon fibers which may be of the GP-grade or HP-grade as a reflection of the difference whether the pitch for spinning is an optically isotropic pitch or the mesophase pitch. While it is essential that the pitch for spinning is a mesophase pitch in order to prepare HP-grade carbon fibers from pitches as the starting material, accordingly, the pitch therefor must be a specific one and the mesophase pitches prepared from ordinary pitches cannot or can hardly be spun into pitch fibers. The specific pitch materials suitable for the preparation of a spinnable mesophase pitch include the pitch obtained from tetrabenzophenazine, residual tars from the high temperature cracking of naphtha or crude oils at about 2000.degree. C. and residual tars from the catalytic cracking of naphthas and the like in the FCC process. The availability of these pitch materials is, however, limited.
Pitch materials of good availability in large quantities such as coal tar pitches and residual pitches by the thermal cracking of naphtha are, however, not suitable as the starting material for a pitch for spinning, as is mentioned above, even when they are converted to a mesophase pitch by a mere pretreatment with heating. Therefore, several improvements have been proposed for the pretreatment of such pitch materials. One of the typical improved methods is the hydrogenation treatment disclosed, for example, in Japanese Patent Kokai No. 57-88016 according to which the starting pitch is subjected to a hydrogenation treatment under pressurized hydrogen together with an aromatic oil in the absence of a catalyst followed by a prolonged heat treatment at about 400.degree. C. to form the mesophase. It is also proposed in Japanese Patent Kokai No. 58-18421 that a hydrogenated pitch is subjected to a heat treatment for a short time at 450.degree. C. or higher under atmospheric or reduced pressure. In this method, the premesophase as a precursor of the mesophase is formed by the combination of the hydrogenation treatment in the first step and the short heat treatment at a high temperature in the second step and characteristic in that the pitch for spinning may not be a mesophase pitch. The premesophase pitch is optically isotropic when it is in the state of the pitch for spinning or the pitch fibers while it is imparted with optical anisotropy when the pitch fibers are carbonized by calcination.
Similarly to the above, an alternative method is proposed in Japanese Patent Kokai No. 57-100186 in which the material at the stage of the pitch for spinning is not necessarily a mesophase pitch. Such a pitch material is called a latent anisotropic pitch obtained by first heat-treating a starting pitch to form a mesophase followed by a reduction with hydrogen by use of ethylenediamine and lithium.
As is understood from the above description, one of the key problems in the technology of carbon fiber production is the preparation of a mesophase pitch or a similar pitch suitable to spinning with ease.
Although the preliminary hydrogenation of the starting pitch provides a very efficient means for the preparation of a pitch of easy spinning along with the broader versatility of the method in respect of the types of the starting pitches, the method unavoidably involves a problem of increased production cost of the carbon fibers and loss of the advantage of the low cost of the starting pitch. It has been recently proposed in this regard that a hydrogenated pitch is blended with an unhydrogenated pitch and the blend is heat-treated for a short time at 450.degree. C. or above so that the hydrogen consumption in the hydrogenation treatment of the starting pitch can substantially be decreased without affecting the spinnability of the pitch for spinning prepared therefrom. This method provides a possibility of decreasing the hydrogen consumption to 50% or less of the conventional methods since the unhydrogenated pitch can be blended with the hydrogenated pitch in an amount of up to 50% of the latter and a possibility of decreasing the investment for the hydrogenation facilities as a result of the decrease in the amount of the pitch to be hydrogenated. At any rate, however, no method is known in which the hydrogenation treatment can be omitted at all.